WatcherImpl<T> class

Implementation of Watcher that observes changes to reactive sources and executes a callback.

This is the concrete implementation of the Watcher interface. Watchers are similar to effects but provide more control over when they trigger. They compare old and new values and only execute when values actually change (or when a custom condition is met).

See Watcher for the public interface and usage examples.

Example:

final count = Signal(0);
final name = Signal('Alice');

final watcher = Watcher(
  () => [count.value, name.value], // Sources to watch
  (newValues, oldValues) {
    print('Count: ${newValues[0]}, Name: ${newValues[1]}');
  },
);

count.value = 1; // Triggers watcher
name.value = 'Bob'; // Triggers watcher